The invention relates to a telecommunication system and to a telecommunication method for a network comprising central stations and wherein terminals communicate with the network through one central station.
It relates more particularly, but not exclusively, to the "Universal Mobile Telecommunication System" (UMTS).
In a system of this kind, the territory of the network is divided into zones, or cells. A central, or base, station is provided for each cell and the terminals, such as mobile telephones, can communicate through the central station.
In these systems, the radio resource management is complex, because terminals inside one zone must not interfere with each other and there must not be any interference between terminals in different zones. Several technologies are known for the sharing of radio resources: for instance, in the GSM system, the resources are distributed in frequencies and in time. The distribution in time is called "time division multiple access" (TDMA); in this case, two different communications using a same frequency resource are separated in time.
Another way of sharing the resources is to use the "CDMA" technique. In this case, a plurality of communications can simultaneously (at the same time) use the same frequency.
CDMA stands for "Code Division Multiple Access". In this case, each user is allocated a code which consists essentially in a spread spectrum sequence such as a Hadamard (or Walsh) sequence, wherein all the sequences are orthogonal. To this sequence is superimposed a scrambling or pseudo noise (PN), generally a Gold sequence.
When the central station transmits to terminals (downlink transmission) messages coded in this way, each terminal receives its own message together with all the others. This terminal can recover its own message because it multiplies the received signals by its own code, i.e. its own spread sequence, and, therefore, the other messages are eliminated due to the orthogonality between the codes.
In such a downlink communication, between the base and the terminals, the PN sequence is used to avoid interference between the corresponding zone and the neighboring zones; in fact, different scrambling sequences are used for the neighboring zones. Moreover, with such scrambling it is possible to take advantage of the multipath diversity.
For uplink communications, i.e. communications transmitted from terminals to the central station, CDMA is also used to distinguish communications coming from different terminals. However, spread spectrum sequences transmitted by two different terminals are generally not orthogonal because they do not reach simultaneously the central station; in fact, it is well known that the orthogonality between Hadamard sequences can be obtained only when there is an accurate synchronism between such sequences. For such uplink communications, different scrambling sequences are used to distinguish communications originating from different terminals.
However, for these uplink communications, orthogonal spread spectrum sequences are used in order to make the distinction between communications transmitted by the same terminal. For instance, a Hadamard sequence is allocated to the information itself and another Hadamard sequence is allocated to control signals such as power or pilot controls. Some terminals may also be able to transmit simultaneously several kinds of information signals, such as speech, data, fax, Internet, video, etc. These communications of different natures are provided with orthogonal Hadamard sequences. As they originate from the same terminal, they arrive simultaneously at the central station and, therefore, can be easily discriminated due to their orthogonal codes.
It is also known to synchronize uplink communications in order to distinguish them by their orthogonal codes. But this synchronization is not currently used, because it is relatively complex to realize.
CDMA telecommunication systems face the difficult problem of elimination of interference for receivers, because all communications received simultaneously strongly interfere with each other. To solve this problem for uplink transmissions, at the central station, conventional rake receivers are used which detect the signals having the correct scrambling or PN sequence and which can cancel the effects of multipath transmissions. But this kind of processing is generally insufficient for CDMA due to the strong interference effect of other communications. This is more particularly true when the number of simultaneous users, or communications, is high. It must also be noted that the signals which have the most detrimental interfering effect are those which have the highest power.
It is the reason why, at the central station, it is preferable for each user, to take into account the effect of other users and to subtract the interfering communications, for instance the most powerful communications, from the communication to be detected.
One straightforward embodiment of this method is to detect the most powerful signals prior to subtracting their contribution to the remaining users.
In order to limit the expenditure. it may be possible to limit the amount of interference detections, but this limitation entails a limitation of interference cancellations. In this situation, in order to obtain the required quality of communication, it is necessary to increase, through a control by the central station, the transmission power of the least powerful terminals. This may happen, for instance, when the number of high bit rate users (which have the most powerful transmitters) increases with respect to the number of telephone (speech) users having a lower bit rate transmission, and therefore, a lower power. But, this increase of power is not always possible because of the limited power availability in the terminals. Anyway, it is generally preferable to decrease the amount of power used by each terminal.